(1) Field of the Invention
This invention relates generally to the field of DC-to-DC converters and relates more specifically to linear and switched DC-to-DC converters having a dynamic automatic input current limit control.
(2) Description of the Prior Art
DC-to-DC converters receive usually DC power from a source comprising a voltage source and an internal resistance. The resistance of a power source, comprising internal resistance and external resistance including the resistance of cables, is often unknown and may often not be neglected. Strong input currents of DC-to-DC converters can cause a substantial voltage drop across this internal resistance.
Every modern integrated power management system has to be able to accommodate for a broad range of voltage sources (USB, 5V wall adapter, Firewire, automotive battery). Each of them comes in a variety of output specs, in particular regarding nominal output voltage and maximum current capability. The power management unit (PMU) has to guarantee that in every circumstance the load seen by the power source is within the specified ranges. This is generally done imposing a current limitation on the PMU according to the kind of power source connected to it.
Even respecting this limitation, an excessive length of the connection cable or a low quality power source can lead the PMU input voltage to fall below a lower threshold specified by the specific power source. In this case the power path between the power source and the system is disabled, as shown in FIG. 1 prior art. Here a general battery operated system has been represented as an ICHG load (current required to charge the battery) and an ISYS load (current required for system operation of an electronic device).
Many DC-to-DC converters use a voltage comparator 1 to detect an input power source. In case an input voltage is higher than a threshold reference voltage an input power source is identified and subsequently an input current is admitted via a switch. In case a substantial voltage drop, caused by a the resistance of a power source (Rout+Rcable) and a strong input current, can be so high that the input voltage Vin is lower than a threshold reference voltage and the input current is switched off. Subsequently without voltage drop the input voltage increases, an input voltage is admitted again and the input of the DC-to-DC converter starts to toggle, which is not acceptable.
The behavior of an architecture shown in FIG. 1 prior art is not robust: even a load current below the current limit can interrupt the power flow and cause start-up or operation failures. This situation would occur for example when charging the battery with full current (i.e. 1 A) connecting the power supply with long cables having a resistance in the order of Ohms.
There are patents or patent publications dealing with the operation of DC-to-DC converters:
U. S. patent (U.S. Pat. No. 7,262,585 to May) discloses a power supply system having a transistor, a linear regulator, a DC-DC converter, and a control circuit. The transistor has an input, a substrate, a first node, and a second node. The first node is operably coupled to a non-battery power source. A linear regulator is operably coupled to the second node to produce a regulated output voltage based on the non-battery power source, when enabled. A DC-DC converter is operably coupled to produce the regulated output voltage based on a battery power source, when enabled. A control circuit is operably coupled to the input node and the substrate of the transistor wherein when the DC-DC converter is enabled, the control circuit controls a reverse leakage current of the transistor, and when the linear regulator is enabled in a zero load-state, the control circuit controls a forward leakage current of the transistor, and when the linear regulator is enabled in a non-zero load-state, the control circuit provides a current limit for the linear regulator.
U.S. patent (U.S. Pat. No. 7,254,044 to Perry et al.) proposes various embodiments of a power supply all including at least one DC/DC converter. The converter includes a primary switch controlled by a pulse width modulated control signal such that the primary switch is on for a D time period of each switching cycle of the converter and is off for a 1-D time period of each switching cycle. Also, the power supply includes a current sensing element connected in series with the primary switch. In addition, the power supply includes a current limit circuit connected to the current sensing element. The current limit circuit includes a functional circuit having a first input responsive to a first signal whose voltage is proportional to the output current of the converter during the D time period of the switching cycle of the converter. A second input of the functional circuit is responsive to a second signal whose voltage is proportional to the output current of the converter during the 1-D time period of the switching cycle of the converter. In that way, the voltage of the output signal of the functional circuit is proportional to the output inductor current of the converter over both the energy storage phase (the D interval) and the energy deliver phase (the 1-D) interval of the converter.
U.S. Pat. No. (4,263,644 to Zellmer) discloses a switched DC-to-DC converter in a power supply being powered by input line current from an external power source and driven by voltage pulses from a variable duty cycle pulse width modulator for converting a DC input voltage to a DC supply voltage of a different value that is applied to a load impedance. A comparator monitors the supply voltage for producing an error voltage that biases the modulator for adjusting the width of the voltage pulses, and thus the duty cycle of the converter, for maintaining the supply voltage relatively constant. An RC circuit integrates the voltage pulses for producing an indication of the average value thereof, which is directly related to the value of line current drawn by the converter. When the average value of voltage pulses exceeds a reference voltage, the value of bias voltage is limited for establishing the maximum width of voltage pulses and duty cycle of the converter, and thereby limit the maximum line current drawn by the power supply.
U.S. Pat. No. (7,414,377 to Mayhew et al.) describes a motor controller system comprising solid-state switches for connection between an AC line and motor terminals for controlling application of AC power to the motor. A sensor senses AC line voltage. A control circuit controls operation of the solid-state switches. The control circuit ramps switch current during a start mode and selectively holds switch current during the start mode if sensed voltage drops below a threshold amount.
Furthermore Texas Instruments has published an application note “Fully Integrated Switch-Mode One-Cell Li-Ion Charger with Full USB compliance and USB-OTG support” describing a charge management device for single cell batteries, wherein charge parameters can be programmed through an I2C interface. The bQ24150/1 charge management device integrates a synchronous PWM controller, power MOSFETs, input current sensing, high accuracy current and voltage regulation, and charge termination, into a small WCSP package.